If you are going to do the Fantasy style then I would ask are you going to include magic? If so, then a mage, dragon, goblin, etc would be a nice addition. If you are going with the SciFi then you need some ships and a space station.

I'll tell you something I've noticed about web comics and comic strips in general - comics based on jokes are never funny. The only thing that really sells and has lasting comedy is funny situations between funny characters. When a joke works, it's not because the joke is funny, it's because one of the characters has a reaction to the joke that's funny.